Mechanistic studies on tandem cascade [4þ2]/ [3þ2] cycloadditionof 1,3,4-oxadiazoles with olefins
The mechanism of the reaction of 1,3,4-oxadiazoles with alkenes (ethylene) and cycloalkenes (cyclo-butene, cyclopentene, cyclohexene and cycloocene) have been studied computationally at the DFT M06e2X/6-311G* level. The reaction is found to proceedviaa concerted [4þ2] addition followed by nitrogenextrusion and then [3þ2] addition in a tandem cascade fashion, which in the case of cycloalkenes leadsto exo-fused or endo-fused subframes, the exo of which is kinetically and thermodynamically favored.The [4þ2] step is the rate-determining step of the reaction. CF3as a substituent on the 1,3,4-oxadiazoledecreases the activation barriers of the rate-determining step, while CO2Me on the oxadiazole increasesthe activation barriers of the rate-determining step, markedly in the case of the reaction with cyclo-pentene and only marginally in the reactions with ethylene. Increasing temperature decreases the barrierof the rate-determining step and stability of the products but increases the rate of the nitrogen extrusionstep. The low barriers of the second and third steps of the reaction compared to thefirst step means thatthe intermediates will not be isolated in the reaction, confirming the experimental observations of earlierworkers. Based on calculated activation barriers, the reactivity of the various cycloalkenes considered inthis study follows the order: cyclooctene>cyclopentene>cyclohexene>cyclobutene which isconsistent with the trends in product yields obtained in earlier experimental studies.
An article published by Elsevier Inc. and also available at https://doi.org/10.1016/j.jmgm.2019.107452
Tandem, 1,3,4-Oxadiazole, Polycyclic molecule, Stereoselectivity, 7-Oxabicylo [2.2.1] heptane
D. Roland et al. / Journal of Molecular Graphics and Modelling 93 (2019) 107452. https://doi.org/10.1016/j.jmgm.2019.107452